Predictably, you have more than a few options here.
If you're happy with your Mackie and want to use your Mac as a digital recorder, you simply need to buy an interface - PCI or FireWire - to record the outputs from your Mackie.
(One question. The Mackie is 8 bus, but you say you want 12 inputs - are you using direct outs from the Mackie input channels in addition to the subgroup outs?)
Assuming you're happy with DP, an interface that works well with it would include MOTU's own FireWire based 828 giving you 10 input channels, and a highly expandable system built around MOTU's HD192 would offer 12 channels (for starters). Money doesn't seem to be an issue, so go for it!
A system like this would give you very high quality recordings on your Mac, but if you really want to mix entirely on the Mac you could consider a slightly different approach with something like the Yamaha 01X. This little digital mixer plugs into your Mac via a FireWire cable, offers 8 analogue inputs, and also functions as a remote control surface for DP (and other sequencers). Additionally, you can access the 01X's onboard multi-fx from your sequencer without leaving the digital domain. In other words, something like the 01X provides a totally integrated solution - automation, fx, remote control, but may be a little limited in terms of the number of simultaneous inputs you can record. Perhaps, when the 01X gets a big brother, the choice will be clear :-)
Small point, there are currently some minor issues with the MOTU drivers for the 424 card (that interfaces the HD192) but MOTU are a good company and will get it fixed soon. There are also issues with the 01X and the MOTU DP - a new driver is imminent too.
One thing's for sure, when you get a system that is solid and does what you want, stick with it and get what you can from it. Change for change's sake means you're forever waiting for bug-fixes!